1. Field of the Invention
This invention relates to radio receivers. More specifically, this invention relates to communications modems which incorporate frequency tracking systems.
While the present invention is described herein with reference to particular embodiments and applications, it is to be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings of this invention will recognize additional applications and embodiments within the scope thereof.
2. Description of the Prior Art
Remotely piloted aircraft or drones may be utilized to provide effective reconnaisance of an enemy position at the forward edge of a battle area. In hostile environment, the drones are subject to destruction by counterfire or being jammed by electronic countermeasures. As a result, it has come to be recognized that such remotely piloted aircraft should be equipped with minimally expensive dispensable hardware, sufficiently sophisticated to penetrate the enemy's electronic defenses.
Drones are currently guided by signals transmitted at relatively low data rates. The rate of transmission typically is on the order of a few hundred bits per second or less. When the aircraft is in flight, the transmitted signals experience Doppler shift in the carrier signal which induces drift or frequency offset in the demodulated data signal. With a carrier signal on the order of 10GHz and the drone flying at a velocity of a few hundred miles per hour, the Doppler shift is on the order of the data rates. The Doppler effect thus interferes with the capability of the drone to receive and interpret guidance commands.
Systems which employ phase lock loops (including decision directed loops) to track the carrier frequency are effective in overcoming the effect of Doppler shift. However, such systems are typically limited in operation to coherent signals, i.e., those having long term phase continuity relative to the time constant of the phase lock loop.
These systems would provide adequate frequency tracking were it not for the desirability of hardening the drone against countermeasures. Unfortunately, some very effective counter-countermeasures, particularly frequency hopping, render the received signal substantially noncoherent.
It is therefore desirable to provide an inexpensive communications modem for use in remotely piloted vehicles which is capable of compensating for Doppler shift in the carrier frequency while being compatible with conventional electronic counter-countermeasure signal preprocessing techniques.